This invention relates to a process for producing polymer electrolyte membranes that have sufficiently high resistance to oxidation and heat as well as high enough proton conductivity to be suitable for use in fuel cells, polymer electrolyte membranes produced by the process, and fuel cell membrane-electrode assemblies using such membranes.
Fuel cells using polymer electrolyte membranes feature high energy density, so making use of fuels such as methanol and hydrogen, they hold promise for application as power supplies to mobile communication devices, household cogeneration systems, and automobiles or as convenient auxiliary power supplies. One of the most critical aspects of the fuel cell technology is the development of polymer electrolyte membranes having superior characteristics.
In a polymer electrolyte membrane fuel cell, the polymer electrolyte membrane serves to conduct protons and it also plays the part of a diaphragm which prevents mixing of the fuel hydrogen or methanol with the oxidant air (oxygen). The polymer electrolyte membrane has several requirements to meet: large ion-exchange capacity; sufficient chemical stability of the membrane to allow for prolonged application of an electric current, in particular, high resistance (oxidation resistance) to hydroxide radicals and the like  which are principal factors that contribute to deterioration of the membrane; heat resistance at 80° C. and above which is the cell operating temperature range; and constant and high water retention of the membrane which enables it to keep low electrical resistance. In addition, the membrane which also plays the part of a diaphragm is required to have high mechanical strength and dimensional stability, as well as having no excessive permeability to the fuel hydrogen gas or methanol or oxygen gas.
Early models of the polymer electrolyte membrane fuel cell used a hydrocarbon-based polymer electrolyte membrane produced by copolymerization of styrene as a monomer capable of retaining sulfonic acid groups and divinylbenzene known as a chemical cross-linking agent. However, this polymer electrolyte membrane, being very poor in durability due to low oxidation resistance, did not have high practical applicability and was later replaced extensively by perfluorosulfonic acid-based membranes such as DuPont's Nafion®.
The conventional perfluorosulfonic acid-based electrolyte membranes such as Nafion® have high chemical durability and stability but, on the other hand, their ion-exchange capacity is as small as about 1 meq/g and, due to insufficient water retention, the membrane will dry up, thereby impedes proton conduction or, in the case of using methanol for fuel, there occurs swelling of the membrane or cross-over of the methanol.
If, in order to increase the ion-exchange capacity,  one attempts to introduce more sulfonic acid groups, the membrane, due to the absence of a cross-linked structure in polymer chains, will swell and its strength drops so markedly that it is prone to break easily. Therefore, with the conventional perfluorosulfonic acid-based electrolyte membranes, the quantity of sulfonic acid groups had to be adjusted to small enough levels to guarantee the required membrane strength, so that one could only produce membranes having ion-exchange capacities of no more than about 1 meq/g.
In addition, the perfluorosulfonic acid-based electrolyte membranes such as Nafion® have the problem of involving a difficult and complex monomer synthesis; what is more, the process of polymerizing the synthesized monomers to produce the intended polymer electrolyte membrane is also complex and yields a very expensive product, thereby presenting a major obstacle to realizing a commercial proton-exchange membrane fuel cell that can be installed on automobiles and other equipment. Hence, efforts have been made to develop low-cost, yet high-performance electrolyte membranes that can be substituted for Nafion® and other conventional perfluorosulfonic acid-based electrolyte membranes.
In the pre-irradiation, post-graft polymerization method and the simultaneous irradiation and graft polymerization method which are closely related to the present invention, attempts are being made to prepare solid polymer electrolyte membranes by grafting into polymer membranes those monomers into which sulfonic acid groups can be introduced.  The present inventors previously conducted intensive studies with a view to developing such new solid polymer electrolyte membranes and found that solid polymer electrolyte membranes characterized by a wide range of controllability of ion-exchange capacity could be produced by first introducing a styrene monomer into a poly(tetrafluoroethylene) film having a cross-linked structure by a radiation-induced graft reaction and then sulfonating the polystyrene grafts. The present inventors filed a patent application for the solid polymer electrolyte membrane having such characteristics and a process for producing it (JP 2001-348439 A). However, this polymer electrolyte membrane was such that the polystyrene grafted chains were composed of hydrocarbons, so when it was supplied with an electric current for a prolonged period of time, the grafted chain portion was partly oxidized to cause gradual decrease in the ion-exchange capacity of the membrane.
In order to solve these problems, the present inventors searched for monomeric styrene substitutes and found that a solid polymer electrolyte membrane more durable than the polystyrene grafted chains by at least two orders of magnitude could be produced by a process in which a monomer that was hydrophobic enough to retain sulfonic acid groups was combined with a chemical cross-linking agent that had balance between rigidity and flexibility and then the pre-irradiation, post-graft polymerization method was performed. Based on this finding, the present inventors filed a patent application for the solid polymer electrolyte membrane and a process for producing it (Japanese Patent Application 2005-170798). 
It was also found that in the process of pre-irradiation of a polymer base film comprising a hydrocarbon matrix, a hydrocarbon-fluorocarbon matrix or a fluorocarbon matrix, the active sites (radicals) necessary for initiating graft polymerization were generated and a crosslinked structure was created but at the same time the substrate's decomposition would also take place depending on the difference in chemical structure between bases, as well as on such conditions as the irradiation atmosphere, temperature and pressure; as a result, the inherent characteristics of the base (e.g. mechanical strength, heat resistance, and durability) deteriorated.